1. Field of the Invention
The present invention relates to compositions for and methods of reducing and eliminating injuries to organs and skeletal tissues subjected to ischemic episodes. The invention also relates to organ transplants, and to preconditioning and preservation of organ transplants. The invention relates particularly to compositions useful for limiting free radical injury to such organs and tissues from ischemic episodes.
2. Background Art
Ischemic insult, i.e., the localized deficiency of oxygen to an organ or skeletal tissue, is a common and important problem in many clinical conditions. The problem is especially acute in organ transplant operations in which a harvested organ is removed from a body, isolated from a blood source, and thereby deprived of oxygen and nutrients for an extended period of time. Ischemic insult also occurs in certain clinical conditions such as sickle cell anemia and septic shock which may result from hypotension or organ disfunction. Depending on the duration of the insult, the ischemia can disturb cellular metabolism and ion gradients, and ultimately cause irreversible cellular injury and death.
Reactive oxygen species are natural but undesirable byproducts of cellular metabolic processes in different subcellular compartments and membranes. These radicals are highly reactive and destructive to cell tissue because of the presence of unpaired electrons. These free radical reactive oxygen species include, for example, O.sub.2.sup.-, OH.sup.-, H.sub.2 O.sub.2, NO, and ONOO.sup.-. In normal systems, injury from these reactive species is prevented or minimized by radical scavenging systems, including enzymatic systems such as catalase, CuZn-superoxide dismutase, Mn-superoxide dismutase, and glutathione peroxidase. Non-enzymatic radical scavenging systems are also present in metabolic processes, such as glutathione, vitamin E, and carotene.
Reactive oxygen species reportedly are formed at a greater rate than they can be scavenged by natural radical scavenging systems when blood is reperfused to an area previously exposed to ischemia. The ischemia/reperfusion also has been reported to cause down regulation of antioxidant enzymatic defenses. As a result of these reports, researchers have incriminated reactive oxygen species as a principal component of the pathology which causes cellular injury as a result of ischemic insult.
The events that cause reactive oxygen species to be produced faster than they can be scavenged by radical scavenging systems following an ischemic episode are not well-understood. The damage caused by these reactive oxygen species has, however, been well documented, and includes increases in intracellular calcium, lipolysis, production of free fatty acids and bioactive arachidonic acid metabolites, proteolysis and decreases in levels of cellular phospholipids.
Ischemia is also associated with various clinical conditions, such as septic shock, that do not involve discreet reperfusive episodes. Septic shock as a result of hypotension and organ dysfunction in response to infectious sepsis is one of the major causes of death. The manifestations of sepsis include those related to the systemic response to infection (tachycardia, tachypnea alterations in temperature and leukocytosis) and those related to organ-system dysfunction (cardiovascular, respiratory, renal, hepatic and hematologic abnormalities). Lipopolysaccharide (LPS) of gram-negative bacteria is considered to be the most important exogenous mediator of acute inflammatory response to septic shock. The LPS or endotoxin released from outer membrane of gram negative bacteria results in the release of cytokines and other cellular mediators including tumor nectrosis factor .alpha. (TNF.alpha.), interleukin-1 (Il-1), interleukin-6 (Il-6) and thromboxane A2. Extreme levels of these mediators are known to trigger many pathological events including fever, shock, intravascular coagulation leading to ischemia and organ failure.
Sickle cell anemia is another condition associated with ischemia. Sickle cell anemia is a classical phenotype of herediatry hemoglobinopathy with hemoglobin S instead of normal hemoglobin A. Sickle cell anemia is associated with hypoxia because of decreased oxygen tension with hemoglobin S. This condition leads to systemic hypoxic condition. The viscosity of deoxygenated blood is related to proportion of sickled red cells, capillary stasis and pain crisis.
Researchers have proposed various compounds for mining ischemic insult following reperfusion. Some, such as deferoxamine, allopurinol, catalase, and peroxidase, are reportedly capable of counteracting free radical production. Others, such as superoxide dismutase, are reportedly capable of destroying these radicals. Still others, such as vitamin E and molecules bearing thiol groups such as N-acetyl cysteine ("NAC") and reduced glutathione, are reportedly capable of neutralizing the free radicals. See, e.g., U.S. Pat. No. 5,498,427.
WO 88/05044 discloses the use of nitric oxide compounds for the prophylaxis and treatment of ischemic cell damage during perfusion, preservation, and reperfusion of organs in cases of cardioplegia or organ transplantations. The nitric oxides are preferably employed as stable free radicals in their reduced form. U.S. Pat. No. 4,877,810 discloses the use of the Trolox derivative of vitamin E, instead of superoxide dismutase ("SOD"), for preventing heart tissue damage upon reperfusion following cardiovascular surgery, including heart transplants. All these therapeutic approaches have been less than ideal in preventing ischemia/reperfusion injury. Therefore, there exists a need for better means of combating ischemia reperfusion injury to tissues and organs. There also exists a need for a better means of combatting ischemia caused by diseases and other conditions that are not associated with discreet reperfusive episodes, such as occur in organ transplants.